ext4 crypto: allocate bounce pages using GFP_NOWAIT

Previously we allocated bounce pages using a combination of
alloc_page() and mempool_alloc() with the __GFP_WAIT bit set.
Instead, use mempool_alloc() with GFP_NOWAIT.  The mempool_alloc()
function will try using alloc_pages() initially, and then only use the
mempool reserve of pages if alloc_pages() is unable to fulfill the
request.

This minimizes the the impact on the mm layer when we need to do a
large amount of writeback of encrypted files, as Jaeguk Kim had
reported that under a heavy fio workload on a system with restricted
amounts memory (which unfortunately, includes many mobile handsets),
he had observed the the OOM killer getting triggered several times.
Using GFP_NOWAIT

If the mempool_alloc() function fails, we will retry the page
writeback at a later time; the function of the mempool is to ensure
that we can writeback at least 32 pages at a time, so we can more
efficiently dispatch I/O under high memory pressure situations.  In
the future we should make this be a tunable so we can determine the
best tradeoff between permanently sequestering memory and the ability
to quickly launder pages so we can free up memory quickly when
necessary.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/crypto.c

@@ -71,12 +71,8 @@ void ext4_release_crypto_ctx(struct ext4_crypto_ctx *ctx)
 {
 	unsigned long flags;
 
-	if (ctx->flags & EXT4_WRITE_PATH_FL && ctx->w.bounce_page) {
-		if (ctx->flags & EXT4_BOUNCE_PAGE_REQUIRES_FREE_ENCRYPT_FL)
-			__free_page(ctx->w.bounce_page);
-		else
+	if (ctx->flags & EXT4_WRITE_PATH_FL && ctx->w.bounce_page)
 		mempool_free(ctx->w.bounce_page, ext4_bounce_page_pool);
-	}
 	ctx->w.bounce_page = NULL;
 	ctx->w.control_page = NULL;
 	if (ctx->flags & EXT4_CTX_REQUIRES_FREE_ENCRYPT_FL) {
@@ -317,22 +313,11 @@ static int ext4_page_crypto(struct ext4_crypto_ctx *ctx,
 
 static struct page *alloc_bounce_page(struct ext4_crypto_ctx *ctx)
 {
-	struct page *ciphertext_page = alloc_page(GFP_NOFS);
-
-	if (!ciphertext_page) {
-		/* This is a potential bottleneck, but at least we'll have
-		 * forward progress. */
-		ciphertext_page = mempool_alloc(ext4_bounce_page_pool,
-						 GFP_NOFS);
-		if (ciphertext_page == NULL)
+	ctx->w.bounce_page = mempool_alloc(ext4_bounce_page_pool, GFP_NOWAIT);
+	if (ctx->w.bounce_page == NULL)
 		return ERR_PTR(-ENOMEM);
-		ctx->flags &= ~EXT4_BOUNCE_PAGE_REQUIRES_FREE_ENCRYPT_FL;
-	} else {
-		ctx->flags |= EXT4_BOUNCE_PAGE_REQUIRES_FREE_ENCRYPT_FL;
-	}
 	ctx->flags |= EXT4_WRITE_PATH_FL;
-	ctx->w.bounce_page = ciphertext_page;
-	return ciphertext_page;
+	return ctx->w.bounce_page;
 }
 
 /**

fs/ext4/ext4_crypto.h

@@ -83,8 +83,7 @@ struct ext4_crypt_info {
 };
 
 #define EXT4_CTX_REQUIRES_FREE_ENCRYPT_FL             0x00000001
-#define EXT4_BOUNCE_PAGE_REQUIRES_FREE_ENCRYPT_FL     0x00000002
-#define EXT4_WRITE_PATH_FL			      0x00000004
+#define EXT4_WRITE_PATH_FL			      0x00000002
 
 struct ext4_crypto_ctx {
 	union {